1. Field of the Invention
The present invention relates to a liquid-crystal display system in which a halftone display is presented by applying two voltages to a pixel of a liquid-crystal display panel alternately in successive frames. More particularly, it relates to a liquid-crystal display system which is most suited to present a flickerless halftone display.
2. Related Art
Heretofore, a method of displaying halftone in a liquid-crystal display system has prevented flicker in such a way that the timing for alternately applying two voltages is different for adjacent lines, as stated in the official gazette of Japanese Patent Application Laid-open No. 62-195628. With this method, however, when displaying a specified display pattern in which the halftone display is presented every second line by way of example, the effect of preventing the flicker using different timings as mentioned above will be canceled and so flicker is again incurred.
The prior-art technique will be described in detail with reference to FIGS. 63xcx9c65. In these figures, a black box represents xe2x80x9cdisplay-OFFxe2x80x9d, a hatched box a xe2x80x9chalftone displayxe2x80x9d, and a blank box xe2x80x9cdisplay-ONxe2x80x9d.
FIG. 63 is a diagram showing the display patterns of respective frames in the case where four illustrated lines are all displayed at a halftone level by the use of the prior art (hereinbelow, the patterns shall be called xe2x80x9chalftone patternsxe2x80x9d). Timings for bestowing the display-ON and the display-OFF states are made different for the adjacent lines in such a manner that the odd lines are in the display-OFF state in the odd frames and in the display-ON state in the even frames, whereas the even lines are in the display-ON state in the odd frames and the display-OFF state in the even frames. Thus, the halftone display is presented within a certain area (the four lines in FIG. 63).
FIG. 64 illustrates a display example being the condition which is perceived by the eye when respective frames are successively displayed on an actual display screen. Although the halftone display is presented for all the four lines in the example of FIG. 63, the halftone is displayed only every second line in this example of FIG. 64. FIG. 65 is a diagram showing the display patterns of the respective frames in the case of the display depicted in FIG. 64.
A liquid crystal displays the halftone between white (display-ON) and black (display-OFF) when repeatedly endowed with the display-ON and the display-OFF states alternately in successive frames. However, when adjacent lines are simultaneously in halftone display states, the repetition of display-ON and display-OFF of these lines at the same timings gives rise to flicker. As illustrated in FIG. 63, therefore, the timings are made different between the adjacent lines so as to prevent the liquid crystal display from flickering.
However, in such a case where the odd lines are set at the halftone display and the even lines at display-ON as shown in FIG. 64, the liquid crystal display flickers as seen from FIG. 65 illustrative of the display patterns of the respective frames. More specifically, in the odd frames, the odd lines are in the display-OFF state, and the even lines are in the display-ON state, while in the even frames, all the lines are in the display-ON state, so that only the odd lines repeatedly alternate between display-ON and display-OFF. The prior-art technique mentioned above does not take into consideration the flickering which is ascribable to the interference between the display pattern as shown in FIG. 64 and the timings for alternately applying the two voltages.
Incidentally, each of the official gazettes of Japanese Patent Applications Laid-open No. 3-2722 and No. 3-20780 discloses a method of driving a liquid-crystal display system wherein tone or grayscale display of different brightnesses in several steps is accomplished by setting a plurality of frames as one cycle and then ON-driving pixels over the number of frames, which corresponds to a grayscale level of display data, within the cycle. In this method, a plurality of adjacent pixels (for example, four pixels or eight pixels) are combined into one group, and the display data for stipulating a tone is designated in group units. Such a method, is intended to reduce the flickering of the displays. This technique, however, adopts a so-called areal modulation system designating a tone in plural-pixel unit and is not directly applicable to a system designating a tone (halftone) in single-pixel unit.
An object of the present invention is to provide a liquid-crystal display system which is not restricted to the areal modulation system, and which can present halftone displays with reduced flicker irrespective of display patterns.
A liquid-crystal halftone display system according to the present invention comprises a data driver which accepts liquid-crystal display data corresponding to input display data representing any of display-ON, display-OFF and halftone for every pixel, for one line, and which delivers the liquid-crystal display data for one line as horizontal display data; a scan driver which appoints a line for displaying the horizontal display data; a liquid-crystal panel which displays the horizontal display data as visible information; a line memory in which the input display data are stored for, at least, one line; and halftone display means for generating the liquid-crystal display data to be afforded to the data driver, by the use of the stored contents of the line memory and the input display data; the halftone display means generating ON data in response to the input display data which represents display-ON for a pixel; generating OFF data in response to the input display data which represents display-OFF for a pixel; and generating the ON data and the OFF data alternately in successive frames as halftone data in response to the input display data which represents halftone display for a pixel, and also comparing the input display data of a pertinent line and those of a preceding line for every line so as to invert a phase of changing-over the ON data and the OFF data in accordance with a result of the comparison.
Another liquid-crystal halftone display system according to the present invention comprises a data driver which accepts liquid-crystal display data corresponding to input display data, for one line, and which delivers the liquid-crystal display data for the one line as horizontal display data; a scan driver which appoints a line for displaying the horizontal display data; a liquid-crystal panel which displays the horizontal display data as visible information; and halftone data generation means provided in correspondence with each at least two of a plurality of tones expressed by the input display data, for delivering first data and second data as the liquid-crystal display data for one pixel alternately in successive frames, wherein a phase of changing-over the first data and the second data is made different for every pixel or for every plurality of pixels and for every line or every plurality of lines.
In operation, with the first liquid-crystal halftone display system of the present invention, the halftone display is presented using the liquid-crystal panel which is capable of ON/OFF (binary) control in pixel units. To this end, the input display data (requiring at least 2 bits per pixel) which represents any of display-ON, display-OFF and halftone states, is received for every pixel, thereby attaining a ternary display brightness per pixel. More specifically, in presenting the halftone display, the halftone display means generates the ON data for the input display data which represents the display-ON of the pixel, generates the OFF data for the input display data which represents display-OFF of the pixel, and generates the ON data and the OFF data alternately in successive frames as the halftone data, for the input display data which represents halftone display of the pixel. Moreover, regarding the halftone display pixel, the halftone display means compares the input display data of the pertinent line and those of the preceding line for every line so as to invert the phase of changing-over the ON data and the OFF data in accordance with the result of the comparison. The phase of changing-over the ON data and the OFF data includes two phases; the first phase in which the data are changed-over in the sequence ON, OFF, ON, OFF, . . . in successive frames with reference to a certain frame, and the second phase-which differs 180 degrees from the first phase and in which the data are changed-over in the sequence OFF, ON, OFF, ON, . . .
More specifically, a signal which repeats ON and OFF alternately for every frame is generated as a halftone reference signal in advance, and the first and second phases are obtained using the halftone reference signal as it is or the inverted signal of this halftone reference signal. As to each of the halftone pixels of the first line in the certain frame (assumed to be, for example, an odd frame), the ON or OFF data is generated in accordance with the phase of the halftone reference signal on that occasion. As to the halftone pixels of the second line et seq. in the same frame, the data of the preceding line are, in principle, inverted. By way of example, when the halftone pixel of the preceding line is ON, the halftone pixel of the pertinent line is turned OFF. Thus, the phases at both the lines become different. In a predetermined case, however, the inversion of the data is inhibited. By way of example, the dot positions and number of the halftone pixels at the pertinent line are compared with those at the preceding line. Then, when the number of those halftone pixels of the pertinent line which differ in the dot positions from the halftone pixels of the preceding line is greater than a predetermined number, the inversion is inhibited. In an even frame, the inverted data of the data of the same lines in the preceding frame are used as the data of the halftone pixels. By way of example, when the halftone data of the same line is OFF in the preceding frame, it is turned ON in the current frame. Incidentally, the phase of the ON/OFF change-over may be made different for the adjacent halftone pixels within one line in such a way that a group of pixels (for example, every other pixels on the line) for which the phase is fixed or unchanged are previously determined on the basis of the dot positions within the line.
In this manner, in the liquid-crystal display control which can appoint the halftone in pixel units, the ON and OFF states are repeated alternately in the successive frames at each of the halftone pixels, and the ON/OFF phases are determined by reference to the display states of the halftone pixels of the preceding line. Accordingly, the ON display states for the halftone display states are prevented from being concentrated in either the even frame or the odd frame, and the liquid-crystal panel is prevented from flickering depending on the display patterns.
With another liquid-crystal halftone display system of the present invention, the halftone display is presented using the liquid-crystal panel which is capable of multiple-valued control in pixel units. When the liquid-crystal pixel is subjected to the multiple-valued control in accordance with the liquid-crystal display data which consists of a plurality of bits per pixel, a ternary or more multiple-valued tone is attained for each pixel. In order to enlarge the number of such tones, the first data and the second data are output alternately in the successive frames as the liquid-crystal display data corresponding to one pixel. In this regard, the halftone display system is characterized in that the phase of changing-over the first data and the second data is made different for every pixel or for every plurality of pixels and for every line or every plurality of lines.
Also in this case, as will be described later, various measures are taken in order that the first and second data for the halftone display in the individual frames may disperse substantially uniformly. Moreover, regarding the relationship between the halftone display and so-called liquid-crystal alternation for an applied liquid-crystal voltage, various expedients are offered from the viewpoint of reducing flicker.
The present invention is applicable, not only to a monochromatic display, but also to a color display, and can realize a flickerless halftone display.